Various constructions for radomes have been proposed in the prior art to provide a protective cover for a microwave antenna. A radome must allow transmission of electromagnetic waves therethrough and also provide the proper structural integrity to protect the microwave system. A radome is an electromagnetic window which can be manufactured into a desired shape and is conventionally used in ground based systems as well as on aircraft, missiles, and other flight vehicles carrying radar or other microwave equipment. In the design of a radome, the offsetting goals of structural integrity and electromagnetic transparency compete and depend upon the particular environment in which the radome is to be used.
Conventionally, radome wall structures have included single-layer wall structures of a homogeneous dielectric constant material wherein the thickness of the wall is designed equivalent to an electrical thickness of 1/2 wavelength or some integer multiple thereof. There have also been thin-wall constructions wherein the thickness of the homogeneous dielectric constant material corresponds to a fraction of the wavelength of electromagnetic energy to be passed through the wall. The thin-wall designs have been found to be suitable for use at very low microwave frequencies where the wavelength of the electromagnetic energy is relatively large. But the resulting walls have had insufficient structural integrity for many microwave applications. A thicker 1/2 wave wall design which provides adequate strength and rigidity allows transmission of electromagnetic energy within a relatively narrow bandwidth to which the radome is tuned, but electrical performance quickly degrades at frequencies above and below the tuned wall thickness of 1/2 wavelength or an integer multiple thereof.
In many applications, the radome must allow transmission of electromagnetic energy over a broad bandwidth. The single-layer design using a homogeneous dielectric constant material has resulted in problems in that the material used in their construction are resonant to a given frequency for which it is designed. This causes the transmission and other electrical properties to degrade when the thickness causes an out of resonance condition. The degradation to electromagnetic energy transmission can be reduced by reducing the material thickness of a thin wall radome because the material is reduced to a lower percentage of the wavelength of the energy being passed therethrough, but creates a problem of providing the necessary structural rigidity for desired applications.
There exist various multi-layer radome constructions which offer enhanced bandwidth and structural capabilities. Some of the same considerations with regard to single-layer radome construction must be considered with respect to a multi-layer construction such as the associated resonance phenomena which is dependent upon the thickness of the layers of dielectric constant material. In the multi-layer construction the material thickness can be reduced to thereby reduce the degradation of the transmission capabilities due to the fact that the thickness of the materials are reduced to a lower percentage of the wavelength of the RF energy being passed therethrough. As an example, a conventional "broadband" radome designed to pass frequencies from DC to 18 GHz presents problems in that the radome wall goes in an out of resonance and the wavelength of the upper frequency range is relatively short. Thus, the thicknesses of the layers in a multi-layer construction are normally selected as the minimum thicknesses which meet the structural requirements for a particular application. Presently, radomes can be adequately constructed to meet both the electrical and structural needs in most applications for frequencies between DC and 18 GHz.
In the typical multi-layer construction for a radome, a mix of low dielectric constant and higher dielectric constant materials is utilized. Due to the fact that the low dielectric constant materials do not have the compressive and flexural strength and stiffness which is necessary for most applications, the higher dielectric constant materials with better mechanical properties enable sufficient structural rigidity to be obtained in the construction. In this way, both the structural and electrical needs are met for applications principally in a range from DC to 18 GHz. The multi-layer radome constructions do have advantages over the single-layer construction but also have inherent problems associated therewith. For example, the combination of required dielectric constant values for passing the desired frequency range and still maintaining the required thickness for structural rigidity is somewhat difficult and inefficient with known materials having the desired dielectric constant properties, especially when working with frequencies above 18 GHz.
Some specific examples of multi-layer designs include the A, B and C sandwiches known in the prior art. An A-sandwich comprises two high dielectric constant material layers which are on both sides of a low dielectric constant material core. The A-sandwich enables electromagnetic energy transmission over a specified frequency range but is susceptible to signal degradation at higher frequencies (such as above 18 GHz) or steep incident angles, when there is sufficient material to be structurally adequate.
Another multi-layer configuration is termed the B sandwich which comprises lower dielectric constant materials on both sides of a high dielectric constant core. Although the B-sandwich is superior electrically to the A-sandwich in the higher frequency range and is relatively broadbanded, the B-sandwich generally does not perform well outside this range (both below and above).
Still another configuration is found in the C-sandwich which comprises a core high dielectric constant material bounded by two low dielectric constant layers which themselves are bounded by yet another layer of high dielectric constant material. The C-sandwich construction is generally used where extreme structural rigidity and broadband capabilities are essential. Although better structural rigidity is obtained with a C-sandwich, limitations still exist on the bandwidth characteristics of the construction.
Another multi-layer construction is shown in U.S. Pat. No. 4,613,350 which utilizes an odd number of three or more layers of reinforced PTFE material. The dielectric constant of each layer is designed to be equal to the square root of the product of the dielectric constants of the two bordering layers in this construction. The electromagnetic window designed in this way was found to function over a predetermined transmission frequency range, which as set forth in the examples therein, is in the range of 3 to 12 GHz or similar frequency ranges. Various other radome constructions can be found in U.S. Pat. Nos. 4,725,475, 4,677,443, 4,358,772 and 3,780,374.
U.S. Pat. No. 4,783,666 discloses a protective shield for an electrically steered, high performance C band antenna array in which a sandwich is formed between two fiberglass layers and a central foam core. An additional interior foam layer is provided for structural support and an additional external fumed silica teflon layer is provided for rain protection. The two foam layers have dielectric constants of 1.07 and 1.04 which means that they have no substantial electrical effect upon the system. The outer teflon layer is a paint only 0.009" thick and thus it has little physical strength and adds no substantial structural rigidity to the final shield.
Although these examples of radome constructions have been found to be suitable over designated frequency ranges of DC to 20 GHz or a relatively narrow bandwidth of higher frequencies, many applications which are presently being developed require much higher frequencies and also much greater broadband characteristics. For example, in many applications the transmission characteristics of the radome are desired to include frequencies of up to 100 GHz or above. It should be recognized that at extremely high frequencies in the range of 100 GHz, the wavelength of the electromagnetic energy is extremely small and the offsetting electrical and structural needs are in sharp contrast.
Based upon the foregoing, it is a main object of the present invention to provide a radome or electromagnetic window construction which may be suitably formed to any shape or size to enable adequate protection of an antenna which is radiating or receiving electromagnetic energy in the range from DC to 100 GHz or above.
It is another object of the invention to provide a radome construction which preserves the transmission characteristics over a large bandwidth, such as DC to 100 GHz, with minimum aberration or distortion and maximum efficiency.
It is a further object to provide an extremely broadband radome construction which is not extremely sensitive to incident angle deviations or polarization effects.
It is yet another object of the invention to provide a novel radome construction which has extremely broadband transmission characteristics compared to prior art and continues to be resistant to thermal stresses as well as erosion due to impact of rain, dust or other particles. Thus, the radome construction is usable either as a ground-based system or under aerodynamic loads in use with flight vehicles or the like.
Another object is to provide good performance over a wide angular range of incident electromagnetic energy.